One engine system being developed for controlled auto-ignition combustion operation comprises an internal combustion engine designed to operate under an Otto cycle. The engine, equipped with direct in-cylinder fuel-injection, operates in a controlled auto-ignition mode under specific engine operating conditions to achieve improved engine fuel efficiency. A spark ignition system is employed to supplement the auto-ignition combustion process during specific operating conditions. Such engines are referred to as Homogeneous Charge Compression Ignition (hereinafter ‘HCCI’) engines.
An HCCI engine operating in HCCI combustion mode creates a charge mixture of combusted gases, air, and fuel in a combustion chamber, and auto-ignition is initiated simultaneously from many ignition sites within the charge mixture during a compression stroke, resulting in stable power output, high thermal efficiency and low emissions. The combustion is highly diluted and uniformly distributed throughout the charge mixture, resulting in low burnt gas temperature and NOx emissions typically substantially lower than NOx emissions of either a traditional spark ignition engine, or a traditional diesel engine.
HCCI has been demonstrated in two-stroke gasoline engines using conventional compression ratios. It is believed that the high proportion of burnt gases remaining from the previous cycle, i.e., the residual content, within the two-stroke engine combustion chamber is responsible for providing the high mixture temperature necessary to promote auto-ignition in a highly diluted mixture.
In four-stroke engines with traditional valve means, the residual content is low and HCCI at part load is difficult to achieve. Known methods to induce HCCI at low and part loads include: 1) intake air heating, 2) variable compression ratio, and 3) blending gasoline with ignition promoters to create a more easily ignitable mixture than gasoline. In all the above methods, the range of engine speeds and loads in which HCCI can be achieved is relatively narrow. Extended range HCCI has been demonstrated in four-stroke gasoline engines using variable valve actuation with certain valve control strategies that effect a high proportion of residual combustion products from previous combustion cycle necessary for HCCI in a highly diluted mixture. With such valve strategies, the range of engine speeds and loads in which HCCI can be achieved is greatly expanded using a conventional compression ratio. One such valve strategy includes trapping and recompression of exhaust gases by early closure of the exhaust valve during the exhaust stroke. Such valve control can be implemented using variable cam phasers; however, cam phaser authority does have adjustment limits.
However, even within the limits of cam phaser authority, in such valve control strategies, low load HCCI engine operation is limited by the combustion chamber temperature achievable. Some extension of low load operation by introducing a first fraction of fuel late during an exhaust stroke of the piston into a combustion chamber including recompressed exhaust gases has been achieved. Such fuel fraction undergoes partial oxidation or reforming reaction to produce additional heat and conditions conducive to auto-ignition of a second fraction of fuel supplied during the compression stroke. However, the amount of fuel that can be reformed in such a manner is limited by recompression temperature and pressure, and oxygen availability. Hence, ultimately, low load HCCI engine operation remains limited by the inability of such techniques to achieve the temperatures necessary for controlled auto-ignition.
Further extension of the low load operating limit has been demonstrated by applying spark assist to the second fraction of fuel injected during the compression stroke. A stratified-ignition assisted, controlled auto-ignition combustion process is realized.